Materials which abrade readily in a controlled fashion are used in a number of applications, including as abradable seals. As will be appreciated by those skilled in the art, contact between a rotating part and a fixed abradable seal causes the abradable material to wear away in a configuration which mates with the moving part at the region of contact. That is, the moving part scrapes away a portion of the abradable seal so that the seal takes on a geometry which precisely fits the moving part. This effectively forms a seal having an extremely close tolerance.
One particular application of abradable seals is their use in turbine engines. Typically, the inner surface of the turbine shroud is coated to a predetermined thickness with an abradable material using a spray gun. In operation, as the turbine blades rotate, they expand somewhat due to the heat which is generated. The tips of the rotating blades then contact the abradable material and carve precisely defined grooves in the coating without contacting the shroud itself . It will be understood that these grooves provide the exact clearance necessary to permit the blades to rotate and thus afford an essentially custom-fitted seal.
In order for the turbine blades to cut grooves in the abradable coating, the material from which the coating is formed must abrade easily without wearing down the blade tips. This requires that a careful balance of materials in the coatings be achieved. In this particular environment, an abradable coating must also exhibit good resistance against particle erosion and other degradation at elevated temperatures. However, as known by those skilled in the art, these desirable characteristics are difficult to obtain using conventional methods of forming abradable coatings.
More specifically, many conventional abradable coatings are formed by plasma spraying the filler and metallic components as a powder, which requires that a number of parameters be carefully monitored. These parameters include the compositional characteristics of the feed powder, powder size, and the various operating conditions of the spray gun. However, even when these factors are closely monitored, conventional equipment and techniques have not been consistently successful in producing high-quality abradable coatings.
In more detail, conventional composite abradable coatings are fabricated by thermal spraying a feedstock selected from two general types. The simplest of these comprises a mixture of a metallic powder and a filler which is usually a non-metallic powder. That is, a blend of the discrete particles of each constituent is prepared which is then sprayed using a plasma spray gun. However, these powder mixtures often segregate, not only in storage, but also in the particle spray stream itself, both of which adversely affect the microstructure of the resultant coating. It is known that particle segregation produces localized regions in the coating consisting predominantly of a single powder constituent. This in turn produces coatings of non-uniform composition and hardness which have inferior serviceability. This lack of uniformity may also be caused by preferential vaporization or other thermal transformation of one of the powder constituents, particularly where a plastic is used as a component. In addition, the use of mixed or blended powders also makes it difficult to adjust the ratio of the constituents to produce graded coatings requiring different blends of feedstock for each layer of the coating.
In the other general class of spray powders, the two constituents are bonded together to form composite particles. A number of bonding techniques are known, such as cladding a first material in powder form with a second material, or by simply bonding two powders together with a suitable binder. However, the binder may not be effective in preventing separation of the two dissimilar materials. Moreover, not only are cladding techniques expensive, but there may also be preferential vaporization of the cladding, which reduces the compositional balance of the coating, and a single powder composition cannot be used to form a coating having different characteristics through the depth of the coating.
For many materials, the production of satisfactory abradable coatings requires the use of extremely high velocities which cannot be achieved with conventional combustion flame spray guns. While plasma spray guns provide high velocities, they operate at such high temperatures that they can cause vaporization and thermal degradation, such as vaporization of the plastic constituent and oxidation of the powder constituents, the latter being accelerated by the turbulence of the spray stream.
Therefore, it would be desirable to provide a method for forming an abradable material by which the problem of particle segregation can be reduced or eliminated. It would also be desirable to provide such a method with the added feature of producing high-quality abradable coatings without producing any significant degradation of the feedstock. It would further be desirable to provide such a method by which a compositional gradient could be attained in a coating by allowing independent control of feedstock constituents without the use of a complex powder metering system and which avoids the steep temperature and velocity gradients of plasma spraying. The present invention provides a method of forming an abradable material which achieves these goals and also provides a novel abradable material formed by the method of the present invention.